设计文法类，实现对文法G[S]=(Vt, Vn, P, S)的文件读写

文法的文件表示形式以及内存表示形式可自定义

文法就是3个集合+1个符号，重点（难点）就是产生式集合

产生式具有结构，可用于推导，是要特别注意的地方

如约定只读写产生式集合，一个示例如下（文件内容）：

S -> a

S -> aSb

也即首个非终结符S为开始符号，然后分析得到里面的终结符集={a, b}，非终结符集={S}，产生式集(略）。

文法的文本形式可根据自己需要自由定义

例如，直接自己给出3个集合。

-Design grammar class, to achieve grammar G [S] = (Vt, Vn, P, S) of document literacy grammar file representation and memory representation can be customized grammar is three set+1 symbols Key (difficulty) is a collection of production with a production structure, can be used to derive, is paying particular attention to areas such as reading and writing production agreement only set an example as follows (file contents): S->　a S->　aSb That is the first non-end symbol S is the start symbol, and then get inside the terminator analysis set = {a, b}, nonterminals set = {S}, production sets (omitted). Text grammar can be freely defined according to their needs, for example, direct yourself to give three sets.